1. Field of the Invention
This invention relates to rubber compositions, and more particularly to a rubber composition having improved cut resistance and chipping resistance by compounding a particular hydroxyl group containing cyclopentadiene resin modified with at least one compound selected from the group consisting of polyisocyanate, polybasic acid, and its anhydride and ester.
2. Related Art Statement
Heavy duty large size tires for truck and bus frequently running on non-paved road, particularly rock-exposed rough road, or off-the-road tires used in stone pites have a frequent chance of subjecting their tread and sidewall portions to cut failure. As a result, it is apt to cause tire burst due to the growth of the cut or due to the corrosion of steel cord reinforcement with rain water penetrated from the cut portion. Therefore, it is required to use a rubber composition having excellent cut resistance and chipping resistance in this type of the tire, particularly its tread portion.
In general, a method of highly filling carbon black is used as a means for improving the cut resistance of the rubber composition. In this case, however, the chipping resistance and heat build-up required in the running on bad road are considerably poor, and also the mixing and extrusion operations are remarkably poor.
On the other hand, Japanese Patent Application Publication No. 48-38,615 discloses that the cut resistance of the rubber composition can be improved by compounding styrene-butadiene copolymer with a cyclopentadiene series resin having a softening point of 50.degree.-200.degree. C. and a bromine number of 40-150. In this case, however, the chipping resistance is still poor though the cut resistance is largely improved.
In order to solve the aforementioned problems, the inventors have proposed in U.S. patent application Ser. No. 12,468 that cyclopentadiene resin modified with 5-alkylidene norbornene-2 or its derivative or cyclopentadiene resin modified with polyisocyanate compound is compounded into a rubber composition. However, the improvement of the cut resistance and chipping resistance is still insufficient in this rubber composition. Therefore, it is very important to further improve the cut resistance and chipping resistance.